1. Field of the Invention
The present invention relates to a beam irradiation device for irradiating laser light onto a targeted area, and more particularly to a beam irradiation device to be loaded in a so-called laser radar system for detecting presence or absence of an obstacle or a distance to the obstacle in a targeted area, based on reflection light obtained by irradiating laser light onto the targeted area.
2. Disclosure of Related Art
In recent years, a laser radar system for irradiating laser light in a forward direction with respect to a driving direction to detect presence or absence of an obstacle or a distance to the obstacle in a targeted area, based on a state of reflection light of the laser light, has been loaded in a family automobile or the like to enhance security in driving. Generally, the laser radar system is so configured as to scan a targeted area with laser light to detect presence or absence of an obstacle at each of scanning positions, based on presence or absence of reflection light at each of the scanning positions. The laser radar system is also configured to detect a distance to the obstacle at each of the scanning positions, based on a required time from an irradiation timing of laser light to a light receiving timing of reflection light at each of the scanning positions.
It is necessary to properly scan a targeted area with laser light, and properly detect each of scanning positions of laser light to enhance detection precision of the laser radar system. As an arrangement for scanning a targeted area with laser light, it is possible to employ an arrangement for driving a mirror about two axes. In the scan mechanism having the above arrangement, laser light is entered into the mirror obliquely with respect to a horizontal direction. Driving the mirror about two axes in a horizontal direction and a vertical direction allows for laser light to scan the targeted area.
In the scan mechanism having the above arrangement, a scanning position of laser light in a targeted area has a one-to-one correspondence to a pivotal position of a mirror. Accordingly, the scanning position of laser light can be detected by detecting the pivotal position of the mirror.
In the above arrangement, the pivotal position of the mirror can be detected by causing servo light to scan over a photodetector (PSD: Position Sensitive Detector) in accordance with movement of the mirror. The pivotal position of the mirror is detected by detecting an incident position of servo light on the photodetector, based on a signal to be outputted from the photodetector, whereby the scanning position of laser light in a target area is detected.
A dark current is superimposed on a signal to be outputted from the photodetector, due to a characteristic of the photodetector. The magnitude of a dark current changes depending on a use state (e.g. an ambient temperature of the photodetector) or a like condition. Since such a dark current may deteriorate position detection precision of the photodetector, it is desirable to properly correct a signal to be outputted from the photodetector by detecting the dark current.
However, since the magnitude of a dark current is considerably small, as compared with a signal to be outputted from the photodetector at the time of turning on of servo light, it is difficult or impossible to properly detect the dark current which is superimposed on a signal to be outputted from the photodetector, depending on an existing circuit configuration, with the result that it may be impossible to obtain a position detection signal free of a dark current with high precision.